1. Field of the Invention
This invention relates to a printing apparatus, and more particularly to a so-called projection type printing apparatus using an imaging optical system to form a mask image on a wafer.
2. Description of the Prior Art
The heretofore known projection type printing apparatuses include a one-shot exposure printing apparatus wherein the whole surface of a mask is exposed en bloc onto a wafer, a scanning type printing apparatus wherein a good image (usually an arcuate image) of part of a mask is projected onto a wafer and the mask and wafer are continuously moved in an orthogonal direction relative to the optical axis of the imaging system whereby the entire image of the mask is exposed onto the wafer, and a step and repeat printing apparatus wherein the image of a mask, on which a circuit pattern usually corresponding to one chip has been recorded, is formed on a wafer by a reduction projection optical system and the wafer is moved in a step-like fashion relative to the mask and the imaging optical system whereby exposure is effected during each movement of the wafer. As imaging optical systems, there are known both one which uses a refraction system (a so-called lens system) and one which uses a reflection system (a so-called mirror imaging system).
In these projection type printing apparatuses, any fluctuation of the spacing between the mask and the wafer which results from any change in room temperature, the nonplanarity of the wafer which results from the warping of the wafer, etc. causes the mask image on the wafer to be out of focus. For this reason, heretofore the spacing between the mask and the wafer has been measured by the use of an air sensor and one of the mask and wafer has been moved in the direction of the optical axis on the basis of the measured value to thereby make the spacing constant.
However, lack of focus of the mask image on the wafer results not only from any variation in the spacing between the mask and the wafer, but also from any fluctuation of the optical characteristic of the imaging optical system which in turn may result from expansion or contraction of the barrel of the imaging optical system due to any change in room temperature. Detection of such lack of focus cannot be accomplished by the use of the aforementioned air sensor.
The only possible way of detecting the lack of focus of the mask image caused by fluctuation of the optical characteristic has been to observe the mask and wafer images at a time through a microscope and detect the condition of their images.